1. Field of the Invention
The present invention relates to a video image coding apparatus for compression encoding a video image, and more particularly to a video image coding apparatus wherein an input image is divided into image divisions and the image divisions are parallelly coded.
2. Description of the Related Art
A video image coding apparatus has made rapid progress particularly in recent years. For example, development of image communication and digital broadcasting which adopt the ISO/IEC13818-2 (MPEG-2 VIDEO) is proceeding at present. For coding of such a standard television signal as is prescribed in the ITU-R.601, a particularly high processing speed is not required, and therefore, an apparatus for the coding can be implemented using a single encoder even with a circuit technique and an LSI technique at present.
However, it is difficult to implement an apparatus for encoding such a video image of a high resolution as a video image of a high definition television (HDTV) broadcast using a single encoder with the circuit technique and the LSI technique at present. In order to solve this problem, it is a common technique to divide an input image signal and parallelly process resulting image division signals with a plurality of encoders. Where the technique just described is adopted, if the encoders are controlled so as to operate equivalently to a single encoder by taking such a method that information is always interchanged between the encoders, then this makes the apparatus and circuit control much complicated and is not practical.
Therefore, it is a method normally taken to control the encoders to perform a coding operation in synchronism with a frame or field period or to control the encoders to operate basically independently of each other. For example, a system wherein coding control is performed independently for each of divided portions of a screen is disclosed in Japanese Patent Laid-Open No. Hei 5-304663 entitled “Image Coding Apparatus”.
However, a property of a video image as represented by a spatial complexity or a magnitude of a movement of an input video image is usually different among different portions of the image. The difference in property of a video image appears as a difference in amount of generated codes or difficulty in coding.
Accordingly, if an allocation information amount which depends upon the bit rate is simply divided equally and is allocated to and encoded by a plurality of encoders which operate independently of each other, then this gives rise to a problem that appropriate information amounts are not allocated to the encoders and a difference in picture quality appears among different divisions of a decoded image. In the worst case, a partial rupture in picture quality or a trouble of a buffer arises.
Further, according to the system disclosed in Japanese Patent Laid-Open No. Hei 5-304663, only when the generated information amount at a certain image division exceeds a threshold value, the quantization characteristic of adjacent image divisions is simply made rough to reduce the picture quality difference of a decoded image. Accordingly, the system described has a problem that appropriate allocation of the information amount of an entire image or efficient utilization of information by providing a comparatively greater amount of information to a portion which requires a comparatively great amount of information but reducing the amount of information at a portion which does not require a great amount of information is not realized.